1. Field of the Invention
The present invention relates to a polyester resin composition which comprises a polyester resin having a cyclic acetal structure and a polyester resin having no cyclic acetal structure, has 0.05% by mole or greater of the sum of the fraction of the dicarboxylic acid unit having a cyclic acetal structure in the entire dicarboxylic acid unit and the fraction of the diol unit having a cyclic acetal structure in the entire diol unit in the polyester resin composition and exhibits excellent transparency, melt viscoelasticity, heat resistance and moldability.
2. Description of the Prior Art
Polyethylene terephthalate (hereinafter, occasionally referred to as PET) is a resin exhibiting an excellent balance between transparency, mechanical properties, solvent resistance, the property to maintain fragrance and the property for recycling and has been used for various applications such as bottles and films. However, PET has drawbacks with respect to crystallization, melt viscoelasticity and heat resistance as shown in the following. With respect to the crystallization rate, when a molded article or a sheet having a great thickness is prepared from PET, the product causes whitening due to crystallization and transparency is decreased. With respect to the melt viscoelasticity, formed articles cannot be prepared from PET by molding due to a small viscoelasticity. With respect to the heat resistance, PET cannot be used for applications requiring a great heat resistance such as materials used for retortable packaging since the glass transition temperature of PET is about 80° C.
For applications requiring small crystallization rate, other polyester resins such as PET modified with 1,4-cyclohexanedimethanol and PET modified with isophthalic acid are used. For applications to formed articles, PET modified with pyromellitic acid or PET modified with glycerol is used. In the field requiring the heat resistance, polyethylene naphthalate and polyarylates are used.
However, PET modified with 1,4-cyclohexanedimethanol, PET modified with isophthalic acid, PET modified with pyromellitic acid and PET modified with glycerol have drawbacks in that controlling the degree of modification is difficult, highly skilled working is necessary for molding to prepare formed articles and the heat resistance is little improve although the crystallization rate and the melt viscoelasticity are improved. Polyethylene naphthalate and polyarylates exhibit about the same crystallization rate and melt viscoelasticity as those of PET although heat resistance is improved. Thus, no polyester resins suitable for applications requiring simultaneous exhibition of a great heat resistance, a small crystallization rate and a great melt viscoelasticity have been known.
On the other hand, as a polyester resin exhibiting a great heat resistance, a polyester resin obtained by modifying PET with 3,9-bis(1,1-dimethyl-2-hydroxyethyl)-2,4,8,10-tetraoxaspiro[5.5]undecane is disclosed in U.S. Pat. No. 2,945,008 and Japanese Patent Application Laid-Open No. 2002-69165. However, in general this resin of the modified PET is great melt viscosity so the modified PET has drawbacks in moldability such as injection molding, sheet molding, etc. although this resin exhibits excellent heat resistance, mechanical properties, transparency, melt viscoelasticity and secondary workability.